The present invention relates to interlaced monitors and, more particularly, to a method of eliminating flicker on an interlaced monitor.
The human visual system retains an image for a fraction of a second after it is viewed. As a result of this “persistence of vision,” a series of still images or frames that are presented at a sufficiently high rate will be integrated by the visual system into a “moving picture.” However, if the time between images exceeds the period of persistence of vision, the image will not be successfully integrated and it will be perceived to flicker. Persistence of vision decreases as the image intensity increases and, therefore, bright images must be displayed more often to avoid flicker. For example, a frame rate of 24 frames per second may be adequate for a relatively dim motion picture suitable for a theater. However, computer monitors which are much brighter than motion pictures typically utilize frame rates of 72 frames per second or greater to avoid flicker.
Generally, computer monitors employ progressive scanning where a complete frame displayed with each scan. Progressive scanning at high frame rates is acceptable for computer monitors because the data bandwidth is relatively unrestricted. However, to limit the required broadcast bandwidth while presenting a relatively flicker free image, television utilizes an interlaced scan format. In the interlaced scan format, an image or frame is displayed by consecutively displaying two fields, each comprising every other scan line of the raster (frame). The fields are displayed at twice the frame rate. At least for images lacking fine detail, the fields contain sufficient information and are displayed with sufficient frequency to permit the visual system to integrate the two fields into a whole image having an acceptable level of “whole image” flicker. Television in the U.S. and monitors conforming to the NTSC format utilize a frame rate of 30 frames per second with a field rate of 60 fields per second. In Europe, a frame rate of 25 frames per second is common for television.
While interlacing is effective in minimizing the bandwidth required for an acceptable level of “whole image” flickering, an interlaced image may include a number of undesirable artifacts that are the result of the fact that interlacing is not truly rapid repetition of the complete image. Vertically adjacent picture elements do not appear at the same time thereby creating jagged edges on moving objects. Horizontal edges may not match in successive scans creating misalignment or interline flickering (a shimmering effect). Flicker is particularly noticeable in images containing narrow horizontal lines such as computer generated “wire frame” drawings, character sets, and crosshatched areas. The sensation of flickering is the result of the high frequency variation of light intensity as the narrow line or sharp edge is repeatedly scanned, and is a function of the level of illumination and the spatial contrast between the line and its surroundings. Horizontal lines and sharp edges are particularly susceptible to flicker because the discontinuity of intensity that is the line or edge is generally parallel to the horizontal scan lines causing the intensity discontinuity to be repeated at the frame rate, not the field rate.
One method of reducing flicker is to perform a low pass filtering operation in the vertical direction. Filtering transverse to the scan lines reduces the local contrast of horizontal edges and eliminates spatio-temporal components of the image signal that could be visible as flicker. However, each image has a different level of detail and, therefore, a different propensity to flicker. Unfortunately, applying a single filter to all images results in loss of detail in images that would exhibit little tendency to flicker. Parulski et al, U.S. Pat. No. 5,428,456, METHOD AND APPARATUS FOR ADAPTIVELY REDUCING INTERLINE FLICKER OF TV-DISPLAYED IMAGE recognize that certain images require more filtering than others and analyze image content before displaying the image to determine the appropriate level of filtering to apply to each image.
Differences in the level and type of detail and, therefore, the propensity to flicker varies within areas of an image as well as between images. Applying a single filter to an entire image can destroy local vertical detail in areas of the image which would not have produced a flickering sensation. Campbell, U.S. Pat. No. 5,019,904, SCAN CONVERTER WITH ADAPTABLE VERTICAL FILTER FOR SINGLE BIT COMPUTER GRAPHICS SYSTEMS, proposes to filter the image with a filter adapted to local conditions. The method is applied to pixels having two states displayed on a progressive scan monitor. Patterns of pixels known to cause flicker are identified by a pattern recognition generator and a filter coefficient is selected for application to the pixels of the pattern on the basis of the propensity of that pattern to cause flicker. Such pattern recognition is complex and computationally expensive, involving evaluation of patterns of pixels in each fixed area (six pixel (high) by three pixel (wide)) neighborhood of an image. Pattern recognition is more complicated and computationally expensive if applied to pixels capable of multilevel intensities. Further, the patterns identified as producing flickering on a progressive scan monitor do not have the same effect when rendered on an interlaced monitor.
What is desired, therefore, is a computationally inexpensive method of detecting and reducing flickering of horizontal lines or edges displayed on an interlaced monitor without unduly degrading image detail.